The objective of this research will be to determine the effects of duration (8 to 20 hr photoperiod), intensity (1-2 to 1000 lux), wavelength (570-640 vs 350-570 vs 500-600 nanometers) and diurnal changes (samples collected throughout the photoperiod and scotoperiod) of daily light exposure on serum prolactin concentrations in cattle. We also plan to study the interactions between photoperiod and ambient temperature (8 C vs 28 C) and between photoperiod and pineal secretions on serum prolactin concentrations. The effects of pinealectomy and administration of various biogenic amines (e.g., L-dopa, serotonin, melatonin, norepinephrine) will be tested for their ability to alter prolactin in vivo and in vitro. Magnitude of prolactin release after a standard challenge with thyrotropin releasing hormone and metabolic clearance rates will be quantified in these studies to determine if it is the release rate or clearance rate which more importantly accounts for the increase in serum prolactin concontrations with increases in length of photoperiod. We also plan to determine if photoperiod affects milk yields in cattle.